In West Virginia’s scenic Canaan Valley National Wildlife Refuge, with its gently sloping mountains and emerald acres of timber, Mike Powell relishes the perks of his job as a caretaker of the land: the sounds of a gurgling stream and the fresh pine scent of evergreens.

But one sight deeply troubles him — the haggard look of the valley’s fabled Christmas trees. Some are bent like old men. The eye-popping green hue that makes people want to adorn them with ornaments had yellowed. A few were covered with hideous waxy balls, a telltale sign that they were under siege by the balsam woolly adelgid, a tiny insect with a notorious reputation among entomologists, who call it “the bug that ate Christmas.”

Along the southern Appalachian range, they are eating two of the nation’s most popular wild Christmas trees — Canaan and Fraser firs — to death.

People who buy Christmas trees at farms need not worry. Farmers who grow Christmas trees control the pest with a potent and costly insecticide, two-man crews spraying one to two acres a day. They work with agricultural extension agents to develop the most efficient pest management strategy because, said Rick Dungey of the National Christmas Tree Association, “it’s very expensive.”

But there’s no stopping adelgids in the wild, where applying chemicals might take out far more organisms than the target.

“We’ve seen a tremendous decline” in the Canaan fir in its native area, said Powell, who manages a tract of woods for the Nature Conservancy. “We’re concerned that it’ll decline to the point that you’ll only see it on tree farms. These trees will survive on tree farms, but in the wild . . . we could lose that tree.”

Fraser firs, North Carolina’s best-selling Christmas tree, have been all but wiped out on Mount Mitchell in Mount Mitchell State Park since the adelgid was discovered there in the mid-1950s.

“The Fraser fir is . . . in peril, badly affected by this adelgid,” said Andrew Liebhold, a research entomologist for the U.S. Forest Service in Morgantown, W.Va.

“I don’t expect them to totally disappear in my lifetime,” Liebhold said. But the die-off of Fraser firs at Mount Mitchell is a major concern. Younger trees are growing there now, “but the danger is when they’re older” and reach 20 feet, a height at which, for unknown reasons, adelgids crave their sap.

Balsam firs, including the Canaan and Fraser varieties, are common in North America, especially from Massachusetts to Maine and the Canadian provinces of Quebec and Ontario. They thrive in cold areas mainly because adelgids struggle to survive in exceedingly cold temperatures.

But farther south, the trees struggle. Climate change could worsen the problem, Powell said, if temperatures warm in the northern range, Massachusetts and much of New England.

Christmas trees in American forests are home to birds, and, in the Canaan Valley, the West Virginia northern flying squirrel, which was recently taken off the endangered species list.

“It’s part of our national heritage, people have gone out to get their own tree for generations, and this is the prize tree they went for,” Powell said.

The balsam woolly adelgid is an uninvited guest in North America, but it’s been around so long that it seems like part of the family. It came from Europe before the 1900s, when there were no regulations guarding against invasive pests, Jill Sidebottom, a forestry extension specialist at North Carolina State University, wrote in an essay last year.

It probably arrived in New England and moved into the southern Appalachians in the 1950s, where the population probably exploded. It “has caused the destruction of the Fraser fir in the natural stands,” Sidebottom wrote.

The same adelgid was detected in San Francisco in 1928 and has chomped through sub-alpine and silver firs in Idaho, Oregon, California and other areas of the Pacific Northwest in large numbers since the 1950s.

Adelgids are about the size of a freckle and about as hard to remove. They are all female and don’t engage in sex. Eggs develop without mating. Two to four generations hatch each year. They have no natural predator.

“It actually feeds on the trunks or large branches of trees,” Liebhold said. Their mouths are like a straw that punctures the bark. “When they’re feeding, they release a chemical in the trees that cause them to deform. When they suck in large numbers, they take nutrients from the trees and they ultimately die.”

The feeding process creates thousands of waxy, woolly balls, which give the adelgid its name. A similar insect, the hemlock woolly adelgid, lays up to 300 eggs in a single woolly ball.

They leave dead shoots and branches, swelling around the shoot nodes known as gouting, a stiff trunk and growth rings with red, hard wood instead of the healthy, creamy white wood, Sidebottom wrote.

Nature has its own way of dealing with problem parasitoids — organisms that kill their hosts — but nature takes its sweet time. “If we wait 100,000 years, the firs here will develop a resistance,” Liebhold said, the way they probably did in Europe.